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In the design of dc-dc converters, there is an ongoing effort to develop devices with higher power density and efficiency. A forward method is widely used as a circuit topology for applications with low output voltage and high output current. However, the fact that an output choke is necessary inhibits miniaturization and cost reduction. To resolve this problem, attempts have been made to integrate the output choke with the transformer. For example, a forward active clamp topology using a four-legged core has been proposed. This circuit operates as a hybrid of forward and flyback converters. The magnetic flux distribution in a transformer core used in this method has been clarified, but a design method has not been established. By means of an analysis, it was found that the magnetic flux in the leg, which acts as a output choke, has both dc and ac components, which are in a trade-off relation. This paper proposes a detailed transformer design methodology that takes account of this relation. When the core of a transformer was designed using the proposed method, it was possible to reduce the footprint of the magnetic device in comparison with that of a conventional converter.